The present invention relates to a liquid developer containing negatively charged toner particles for developing electrostatic charge images, which comprises a carrier liquid of high electric resistivity and low dielectric constant, a pigment or dye, an N-vinylpyrrolidone-containing polymer, a waxy substance and conventional additives, and to a process for its preparation.
Liquid developers of this type are used in electrophotographic or electrographic copying processes in order to render latent electrostatic charge images visible. In principle, these developers are composed of colored particles which are dispersed in a solution of binder and charge control agent, in the carrier liquid, if required, together with a dispersing agent. The colored particles, with the charge control agent and the binder adhering thereto, are electrophoretically deposited in the electric field of the charge image. If an N-vinylpyrrolidone-containing polymer is present, the dispersing agent, binder and charge control agent may be identical.
It is known from German Offenlegungsschrift No. 27 40 870 (equivalent to U.S. Pat. No. 4,243,736) or from European Patent Application No. 0,037,475 to use waxes in amounts corresponding to the proportions of pigments in liquid developers which contain an N-vinylpyrrolidone-containing polymer as the binding and dispersing agent, and at the same time as the charge control agent. The waxes serve to improve the sedimentation properties of the liquid developer and the wipe-resistance of the developed images. Polyethylene wax is the wax employed. Minor amounts of halogenated paraffins can also be present. It has been shown, however, that full shade areas are made visible by these liquid developers, which do not yet satisfy highest performance demands.
Other printed publications, also, propose the addition of waxes to liquid developers. For example, a suspension developer is described in German Offenlegungsschrift No. 25 32 282 (equivalent to U.S. Pat. No. 3,992,342), which additionally contains polyethylene, polyethylene wax, and/or paraffin wax having a softening point of 60.degree. C. to 130.degree. C. By the admixture, the stability, sedimentation properties and viscosity of the liquid developer, the gloss and wipe-resistance of the copies obtained, and the degree of fouling of the apparatus are influenced. German Auslegeschrift No. 25 38 581 (equivalent to U.S. Pat. No. 4,081,391) and German Auslegeschrift No. 29 36 042 (equivalent to U.S. Pat. No. 4,250,241) also describe electrophotographic suspension developers which additionally contain a wax or polyolefin having a softening point of 60.degree. C. to 130.degree. C. The specific weight of these admixtures is similar to that of the carrier liquid. They are soluble in the heated liquid developer and separate off in the form of small particles during the cooling down phase.
German Offenlegungsschrift No. 30 46 654 (equivalent to U.S. Pat. No. 4,306,009) discloses a liquid developer which includes a special gelatex in addition to the customary components and may also contain a wax and a wood resin.
An examination of the respective descriptions shows, however, that only polyethylene, polyethylene wax or paraffin wax and beeswax are employed, such as, for example, a polyethylene having a softening point of 115.degree. C. and a molecular weight of 1,500, low-molecular weight polyethylene having a softening point of 107.degree. C. or 108.degree. C., and paraffin waxes having softening points ranging between 60.degree. C. and 110.degree. C. It is possible to achieve high print runs with some of these liquid developers, but the copies obtained exhibit unsteady, nonhomogeneously inked full shade areas. This is of particular disadvantage in cases where copying paper of low surface smoothness is used.